molekulaarne osake, mis on võimeline loovutama hüdroni (prootoni) alusele, samuti vastav keemiline ühend. Näited: H2O, H3O+, CH3CO2H, H2SO4, HSO4−, HCl, CH3OH, NH3, a molecular entity capable of donating a hydron (proton) to a base, (i.e. a 'hydron donor') or the corresponding chemical species. For example: H2O, H3O+, CH3CO2H, H2SO4, HSO4−, HCl, CH3OH, NH3